To meet the need for ever-increasing information capacity in wireless communication systems, research efforts have recently turned to the physical layer to increase spectral efficiency. One aspect of this research relevant to certain embodiments of this invention is in the area of multiuser receivers. These receivers seek to minimize interference between mutual users of a spread spectrum wireless system, and generally include multiuser detectors, linear decorrelators, and linear minimum mean-square-error (MMSE) receivers. An important distinction between multiuser receivers and their matched-filter counterparts is that multiuser receivers delve into the structure of interference among different (simultaneous) users or the system in order to demodulate the signal of one particular user of interest
Certain prior art receivers increase either information-carrying capacity (traditional capacity, such as defined by the Shannon limit), or user capacity, the latter being the maximum number of users for which a multiuser receiver may reliably demodulate the intended signal. For example, one type of multiuser receiver resolves multiple access interference (MAI) using knowledge of either all (conventional) or none (blind) of the interfering users, though in practice this knowledge in the receiver is incomplete. Another approach is for the multiuser receiver in an ultra-wideband system to combine energy from the multiple users at the densest portion of the multipath channel while disregarding both wideband and narrowband interference. As with much research in spread spectrum wireless communications, these appear to rely on the difference in user power levels being within a quite narrow band.
Regardless of the practical application of the above receivers to cellular communications where effective power control among users is a viable assumption, there exist wireless systems where that assumption does not hold, and where Doppler effects are so great that power control is not practical, in particular in system which transmit data in bursts. Such a system 20 is shown in FIG. 1.
FIG. 1 depicts a series of nodes communicating with one another over a wireless network, preferably secure. Assume a first node 22 carries a multiuser receiver, and wishes to receive communications from any of the other nodes. A second 24 and third 26 node are nearest to the first, and traditional power control may be adequate between them and the first node 22. A fourth 28 and fifth 30 nodes are located proximal to one another as compared to the first node 22, but are moving in different directions at high speed. Doppler effects are opposed and not negligible. Direct power control to the extent achieved in cellular communications is impractical for them, because each node 28, 30 changes its distance from the first node 22 rapidly, and power control cannot keep up with the spatial changes between nodes. As the fourth node 28 closes on the first node 22, its signal carries a higher power level that will obscure the signal from the fifth node 30, whose power level as seen by the first node 22 is decreasing. Traditional open/closed loop power control will lag behind the power levels of the various users seen by the first node 22, essentially blinding it to at least some of the transmissions from the other nodes.
Anticipatory power control based on expected relative position might resolve the above problem were the system 20 a cellular one. Traditional power control presumes that only a single node need receive transmissions from multiple users. In that model individual cellular users can adjust their transmission power such that all received power levels at the cellular base station are equal. But the system 20 of FIG. 1 is a mesh network, and any of the various nodes may include a multiuser receiver to receive transmissions from any other node. In that instance, anticipatory power control may work to constrain received signals to within a narrow power range for one of the nodes, but will expand, even beyond a range that would exist in the absence of power control, the power range for those same signals when they are received by other nodes (depending upon the location of those other nodes).
The above is generally termed in the art as a near-far power imbalance. In the system 20 of FIG. 1, the difference in power for various signals received at the first node 22 may be several orders of magnitude. At any given instant, the first node 22 may seek to receive a signal from what it sees as the strongest fourth node 28 and a weakest seventh node 34. A sixth node 32 may simultaneously seek to receive a signal from what it sees as an intermediate strength fifth node 30 and a weakest second node 24.
In the prior art, coherent multiuser receivers are used to resolve the different user signals. However, they have difficulty in resolving chip and carrier frequency in the presence of large multiuser interference, and often operate reliably only under high signal to noise ratios. What is needed in the art is a multiuser receiver and method that permit detection of multiple spread spectrum users even in the presence of large Doppler uncertainties, large power imbalances, and/or low signal to noise ratios.